Macrophages, granulocytes, many lymphocytes, and certain epithelia all[unreadable] express receptors for the Fc domain of IgG. Fc receptors (FcR) serve[unreadable] diverse functions in cellular and humoral immunity. In macrophages, they[unreadable] mediate the endocytosis soluble antibody-antigen complexes, the[unreadable] phagocytosis of large IgG-coated particles, and the release of potent[unreadable] cytotoxic and inflammatory agents. FcR on B-lymphocytes, on the other[unreadable] hand, regulate the process of B-cell activation via surface Ig.[unreadable] Recently, this diversity in function has been found to reflect an even[unreadable] greater diversity in structure. FcR comprise a large, multi-gene family[unreadable] of Ig-related molecules. Among the most important of these if FcRII, a[unreadable] receptor class expressed on virtually all FcR-positive cells. However,[unreadable] even this single class exhibits considerable structural heterogeneity due[unreadable] to cell type-specific alternative mRNA splicing that generates receptors[unreadable] with different cytoplasmic domains. In murine cells, the major splice[unreadable] products are mFcRII-B1 (lymphocytes) and -B2 (macrophages). These[unreadable] isoforms are identical except for an in-frame insertion of 47 amino acids[unreadable] in the cytoplasmic tail of mFcRII-B1.[unreadable] Over the past few years, we have begun to establish the functional[unreadable] differences between mFcRII-B1 and -B2, and to determine whether different[unreadable] receptor activities are associated with distinct regions of the FcRII[unreadable] cytoplasmic domain. For example, the presence of the insertion[unreadable] completely blocks endocytosis by preventing receptor accumulation at[unreadable] coated pits. In this proposal, we will extend these investigations using[unreadable] a combination of biochemical, molecular, morphological, and genetic[unreadable] approaches. First, we will better define the unique features of the[unreadable] FcRII-B2 coated pit localization domain. Although coated pit[unreadable] accumulation involves a conserved tyrosine-containing region of the[unreadable] receptor's cytoplasmic tail, more detailed analysis is required since[unreadable] unlike most other plasma membrane receptors the tyrosine residue itself[unreadable] is not required for coated pit localization in all cell types. Using[unreadable] molecular and biophysical approaches, the FcRII coated pit domain will[unreadable] be precisely defined and compared to more "conventional" coated pit[unreadable] domains. Second, we will define the mechanism by which the alternatively[unreadable] spliced insertion in mFcRII-B1 (and its human homolog) prevents coated[unreadable] pit localization. Preliminary evidence suggests that the insertion[unreadable] functions by actively preventing coated pit entry, perhaps by binding the[unreadable] receptor to the cytoskeleton. Third, we will define the domains involved[unreadable] in the regulation of B-cell activation. The role of tyrosine[unreadable] phosphorylation will be evaluated since this domain appears to overlap[unreadable] with the region required for coated pit localization, and its tyrosine[unreadable] falls within a consensus tyrosine phosphorylation site. Fourth, we will[unreadable] identify sequences required for FcRII-mediated phagocytosis and signal[unreadable] transduction in transfected fibroblasts and macrophage lines. Finally,[unreadable] we will develop the use of a genetically manipulable professional[unreadable] phagocyte, Dictyostelium discoideum, as a genetic system for the study[unreadable] of phagocytosis in general and FcRII function in particular.[unreadable] [unreadable]